1. Field of the Invention
The present invention relates generally to methods and apparatus for manufacturing optical fiber preforms, and particularly to purifying process gases to remove transition metal impurities therein.
2. Technical Background
Typically, in the process of manufacturing optical fiber, the first step is manufacturing of an optical fiber preform. In such preforms manufactured by for example Outside Vapor Deposition (OVD) methods, the preform first takes the form of a soot blank. The soot blank is formed by depositing layers of silica-containing soot by flame deposition onto a rotating deposition surface, such as a generally cylindrical core cane or mandrel. U.S. Pat. No. 3,933,454 describes a suitable method for forming a soot preform. The soot preform is than dried by placing it into the heated chamber of a consolidation furnace and subjecting the soot preform to a temperature between about 850 and 1150° C. in a drying gas atmosphere. The drying atmosphere typically includes a chlorine-containing gas, such as Cl2. U.S. Pat. No. 4,165,223 teaches an exemplary method for appropriately drying and consolidating a soot preform.
Once dried, the soot preform may be subjected to doping. Doping raises or lowers the refractive index of one or more portions of the preform, as compared to pure silica. Typically preforms may be doped with fluorine, for example, by subjecting the soot preform to a fluorine-containing gas such as CF4, C2F6, C2F2Cl2, SF6 or SiF4 for a sufficient time s fluorine diffuses into the interstices of the preform. U.S. Pat. No. 4,629,485 to Berkey describes a method of making a fluorine-doped optical fiber preform.
Once sufficiently dried and/or doped, the soot blank is subjected to an elevated temperature of between about 1250-1600° C. (depending upon dopant concentration) until the soot blank vitrifies and produces a consolidated glass preform. This consolidated preform is then placed into a chamber of a draw furnace and drawn into optical fiber by conventional methods. Alternatively, the consolidated preform may be redrawn in a furnace to form a cylindrical core cane rod, a segment of which then becomes the deposition surface for further deposition steps before being dried and consolidated again such that optical fiber can be drawn therefrom.
The attenuation of the optical fiber produced from such preforms is particularly important property, with even small attenuation reductions, as low as 0.0001 dB/km, being considered significant. Previous efforts have focused on providing, directly from the manufacturer, raw materials having impurities in the ppb range. However, even given this very low level of impurities, it is still desirable to further improve prior art processes such that optical fiber with even lower attenuation may be produced.